Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals, wireless devices and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular or wireless communications network covers a geographical area which may be divided into cell areas, wherein each cell area may be served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. evolved NodeB “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The 3GPP initiative “License-Assisted Access” (LAA) intends to allow an LTE equipment, such as a communication device, to also operate in the unlicensed 5 GigaHertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum may be used as a complement to the licensed spectrum. Accordingly, communication devices may connect in the licensed spectrum, through e.g., a primary cell or PCell, and may use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum, through e.g., a secondary cell or SCell. To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell may be simultaneously used in the SCell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum may be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) method may need to be applied. The LBT procedure may involve sensing a medium for a pre-defined minimum amount of time, and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
LTE
LTE may use Orthogonal Frequency Division Multiplexing (OFDM) in the DL and Discrete Fourier Transform (DFT)-spread OFDM, also referred to as Single-Carrier Frequency Division Multiple-Access (SC-FDMA), in the UL. The basic LTE DL physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The UL subframe may have the same subcarrier spacing as the DL, and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the DL. The subcarrier spacing has been chosen to be 15 kiloHertz (kHz), as shown. Each resource element may comprise a so-called cyclic prefix, which is involved in preventing inter-symbol interference.
In the time domain, LTE DL transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2, which illustrates the LTE time-domain structure. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame may range from 0 to 19. For normal cyclic prefix, one subframe may consist of 14 OFDM symbols. The duration of each symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE may typically be described in terms of resource blocks, where a resource block corresponds to one slot, 0.5 ms, in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in the time direction, 1.0 ms, may be known as a resource block pair. Resource blocks may be numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions may be dynamically scheduled, i.e., in each subframe the base station may transmit control information about which terminals data is transmitted to, and upon which resource blocks the data is transmitted, in the current DL subframe. This control signaling may be typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe, and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The DL subframe may also contain common reference symbols, which may be known to the receiver, and used for coherent demodulation of e.g., the control information. A DL system with CFI=3 OFDM symbols as control region is illustrated in FIG. 3, which illustrates a normal DL subframe. The control region in FIG. 3 is shown as comprising control signaling, indicated by black squares, reference symbols, indicated by striped squares, and unused symbols, indicated by checkered squares.
From 3GPP LTE Release 11 onwards, the above described resource assignments may also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Release 8 to Release 10, only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in the above FIG. 3 are the Cell-specific Reference Symbols (CRS) and they may be used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Physical Downlink Control Channel (PDCCH) and Enhanced PDCCH (EPDCCH)
The PDCCH and/or EPDCCH may be used to carry DL Control Information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI may include:
a) Downlink scheduling assignments, including the Physical DL Shared CHannel (PDSCH) resource indication, transport format, hybrid-Automatic Repeat reQuest (ARQ) information, and control information related to spatial multiplexing, if applicable. A DL scheduling assignment may also include a command for power control of the Physical Uplink Control CHannel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to DL scheduling assignments.
b) Uplink scheduling grants, including Physical UL Shared CHannel (PUSCH) resource indication, transport format, and hybrid-ARQ-related information. An UL scheduling grant may also include a command for power control of the PUSCH.
c) Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.
One PDCCH and/or EPDCCH may carry one DCI message containing one of the groups of information listed above. As multiple terminals may be scheduled simultaneously, and each terminal may be scheduled on both DL and UL simultaneously, there may be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message may be transmitted on separate PDCCH and/or EPDCCH resources, and consequently, there may be typically multiple simultaneous PDCCH and/or EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation may be used, where the code rate of the PDCCH and/or EPDCCH may be selected by adapting the resource usage for the PDCCH and/or EPDCCH, to match the radio-channel conditions.
Here follows a discussion on the start symbol for PDSCH and EPDCCH within the subframe. The OFDM symbols in the first slot may be numbered from 0 to 6. For transmissions modes 1-9, the starting OFDM symbol in the first slot of the subframe for EPDCCH may be configured by higher layer signaling and the same may be used for the corresponding scheduled PDSCH. Both sets may have the same EPDCCH starting symbol for these transmission modes. If not configured by higher layers, the start symbol for both PDSCH and EPDCCH may be given by the CFI value signaled in the Physical Control Format Indicator CHannel (PCFICH).
Multiple OFDM starting symbol candidates may be achieved by configuring a UE in transmission mode 10, by having multiple EPDCCH Physical Resource Block (PRB) configuration sets where for each set the starting OFDM symbol in the first slot in a subframe for EPDCCH may be configured by higher layers to be a value from {1,2,3,4}, independently for each EPDCCH set. If a set is not higher layer configured to have a fixed start symbol, then the EPDCCH start symbol for this set may follow the CFI value received in the PCFICH.
Carrier Aggregation
The LTE Release 10 standard may support bandwidths larger than 20 MegaHertz (MHz). One requirement on LTE Release 10 may be to assure backward compatibility with LTE Release 8. This may also include spectrum compatibility. That may imply that an LTE Release 10 carrier, wider than 20 MHz, may appear as a number of LTE carriers to an LTE Release 8 terminal. Each such carrier may be referred to as a Component Carrier (CC). In particular, for early LTE Release 10 deployments, it may be expected that there may be a smaller number of LTE Release 10-capable terminals compared to many LTE legacy terminals. Therefore, it may be necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it may be possible to implement carriers where legacy terminals may be scheduled in all parts of the wideband LTE Release 10 carrier. The straightforward way to obtain this may be by means of Carrier Aggregation (CA). CA implies that an LTE Release 10 terminal may receive multiple CC, where the CC may have, or at least the possibility to have, the same structure as a Release 8 carrier. CA is illustrated in the schematic diagram of FIG. 4, where 5 carriers of 20 MHz each are aggregated to form a bandwidth of 100 MHz. A CA-capable communication device, such as a UE, may be assigned a Primary Cell (PCell) which is always activated, and one or more Secondary Cells (SCells), which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL is the same, whereas an asymmetric configuration refers to the case that the number of CCs is different. It may be noted that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more DL CCs than UL CCs, even though the cell is configured with the same number of UL and DL CCs.
In addition, a feature of CA may be the ability to perform cross-carrier scheduling. This mechanism may allow an (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE may expect to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH may also be configured semi-statically.
LTE Measurements
A UE may perform periodic cell search and Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) measurements in Radio Resource Control (RRC) Connected mode. It may be responsible for detecting new neighbor cells, and for tracking and monitoring already detected cells. The detected cells and the associated measurement values may be reported to the network. Reports to the network may be configured to be periodic or aperiodic based a particular event.
Rel-12 LTE Discovery Reference Signal (DRS)
To share the channel in the unlicensed spectrum, the LAA SCell may not occupy the channel indefinitely. One of the mechanisms for interference avoidance and coordination among small cells may be the SCell ON/OFF feature, whereby when a small cell has no or low traffic, the small cell may be turned off or dynamically blanked to reduce the interference to neighboring cells. In Rel-12 LTE, discovery signals were introduced to provide enhanced support for SCell ON/OFF operations. A discovery signal may be understood as a set of reference signals and synchronization sequences that may be transmitted together in the same subframe in order to facilitate synchronization, Radio Resource Management (RRM) measurements, and channel estimation. Specifically, these signals may be introduced to handle a potentially severe interference situation, particularly on the synchronization signals, resulting from dense deployment, as well as to reduce UE inter-frequency measurement complexity.
A so called DRS occasion may be understood herein as the time period wherein DRS are transmitted, e.g., from a cell. The discovery signals or Discovery Reference Signal (DRS) in a DRS occasion may be comprised of the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), the CRS and when configured, the Channel State Information Reference Signals (CSI-RS). The PSS and SSS may be used for coarse synchronization, when needed, and for cell identification. The CRS may be used for fine time and frequency estimation and tracking and may also be used for cell validation, i.e., to confirm the cell IDentity (ID) detected from the PSS and SSS. The CSI-RS is another signal that may be used in dense deployments for cell or transmission point identification. FIG. 5 shows the presence of these signals in a DRS occasion of length equal to two subframes and also shows the transmission of the signals over two different cells or Transmission Points (TP).
FIG. 5 is a schematic diagram of the OFDM subcarriers and symbols in two subframes, wherein the dotted RE with light background represent SSS, the dotted RE with black background represent PSS, the striped RE represent CRS, the black RE represent empty RE, and the checkered RE represent CSI-RS. The two subframes are separated by a bold vertical bar.
The DRS occasion corresponding to transmissions from a particular cell may range in duration from one to five subframes for Frequency Division Duplex (FDD), and two to five subframes for Time Division Duplex (TDD). The subframe in which the SSS occurs may mark the starting subframe of the DRS occasion. This subframe is either subframe 0 or subframe 5 in both FDD and TDD. In TDD, the PSS may appear in subframe 1 and subframe 6, while in FDD the PSS may appear in the same subframe as the SSS. The CRS may be transmitted in all DL subframes and Downlink Pilot TimeSlot (DwPTS) regions of special subframes.
The discovery signals may be useable by the UE for performing cell identification, RSRP and RSRQ measurements. The RSRP measurement definition based on discovery signals may be the same as in prior releases of LTE.
In Rel-12, RSRP measurements based on the CRS and CSI-RS in the DRS occasions and RSRQ measurements based on the CRS in the DRS occasions have been defined. As stated earlier, discovery signals may be used in a small cell deployment where the cells are being turned off and on or in a general deployment where the on/off feature is not being used. For instance, discovery signals may be used to make RSRP measurements on different CSI-RS configurations in the DRS occasion being used within a cell, which may enable the detection of different transmission points in a shared cell.
The provision of DRS timing information may be done via a Discovery Measurement Timing Configuration (DMTC) that may be signaled to the UE. The DMTC may provide a window with a duration of 6 milliseconds (ms) occurring with a certain periodicity and timing within which the UE may expect to receive discovery signals. The duration of 6 ms may be the same as the measurement gap duration as defined currently in LTE and may allow the measurement procedures at the UE for discovery signals to be harmonized regardless of the need for measurement gaps. Only one DMTC may be provided per carrier frequency including the current serving frequencies. The UE may expect that the network will transmit discovery signals so that all cells that are intended to be discoverable on a carrier frequency transmit discovery signals within the DMTCs. Furthermore, when measurement gaps may be needed, it may be expected that the network may ensure sufficient overlap between the configured DMTCs and measurement gaps.
Wireless Local Area Network (WLAN)
In typical deployments of WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) may be used for medium access. This means that the channel may be sensed to perform a Clear Channel Assessment (CCA), and a transmission may be initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission may be essentially deferred until the channel is deemed to be Idle. When the range of several Access Points (APs) using the same frequency overlap, this may mean that all transmissions related to one AP may be deferred in case a transmission on the same frequency to or from another AP, which is within range, may be detected. Effectively, this may mean that if several APs are within range, they may have to share the channel in time, and the throughput for the individual APs may be severely degraded. A general illustration on an example of the Listen Before Talk (LBT) mechanism or process is shown in FIG. 6. The LBT procedure may either be frame-based or load-based. The frame based LBT framework may allow an equipment to perform a CCA per fixed frame period for a duration of T1, as illustrated in FIG. 6 by a circled 1. CCA may be performed using Energy detection. If the channel is found to be available after the CCA operation, as indicated by a check sign in the Figure, the equipment may transmit immediately up to the maximum allowed channel occupancy time, for example 10 ms, where this time may be referred to as the channel occupancy time, T2, and denoted by circled 2 in FIG. 6. In the example of FIG. 6, a communication device may perform an extended CCA under the load-based LBT framework, as described for example in Europe regulation EN 301.893 v 1.7.1, load-based LBT procedure. The extended CCA under the load-based LBT framework may also be referred to herein as a complete random backoff procedure, and is indicated in the Figure with a circled 3. Basically, a complete random backoff procedure may be understood as involving drawing a random number prior to transmission, and determining that the channel has been idle for said number of observation slots, i.e., CCA, before commencing transmission. The range from which the random number is drawn may be modified, depending on whether previous transmissions were successful or unsuccessful. During the random backoff procedure, before the equipment transmits for the first time on an operating channel, the equipment may check if the channel is currently idle. At the end of the required idle period, the equipment may resume CCA for channel access. If the channel is not idle, the equipment draws a random number N of CCAs after which the channel has to be available before transmission may occur. N, in FIG. 6 is 3. A counter may be set to 3, as indicated in FIG. 6, and 1 is subtracted from the current N value, every time the channel is observed to be available after a CCA. If the channel is found to be busy after the CCA operation, as indicated by a cross sign, no value is subtracted, or a 0 value is subtracted. When N is counted down to 0, transmission may take place during the second period indicated by “Transmission” in FIG. 6, starting from the left end of the Figure. During the second period indicated by the circled number 2 in FIG. 6, starting from the left end of the Figure, data may be transmitted and control signals may be sent without a CCA check during the period denoted as “Ctrl” by a circled 4.
In contrast to a complete random backoff procedure, a short CCA may be understood as observing the channel for a fixed, short number of CCA slots, such as one slot, without drawing a random number, as described above.
In LAA, described below, DRS that may be transmitted without PDSCH in the same subframe/s may be sent after a short Clear Channel Assessment (CCA), based on a single sensing interval. In other words, a complete random backoff procedure may not be required to be followed when sending DRS without PDSCH.
Licensed-Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE may be dedicated to LTE. This may have the advantage that the LTE system may not need to care about the coexistence issue and the spectrum efficiency may be maximized. However, the spectrum allocated to LTE is limited, which may not meet the ever increasing demand for larger throughput from applications and/or services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum may, by definition, be simultaneously used by multiple different technologies. Therefore, LTE may need to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum may seriously degrade the performance of Wi-Fi, as Wi-Fi may not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably may be to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 7, a UE may be connected to a PCell in the licensed band and one or more SCells in the unlicensed band. Herein, a SCell in unlicensed spectrum may be referred to as a License-Assisted Secondary Cell (LA SCell) or Licensed-Assisted Access Cell. FIG. 7 illustrates LAA to unlicensed spectrum using LTE carrier aggregation.
Further detailed information on some aspects discussed herein may be found in: 3GPP TS 36.211, V11.4.0 (2013-September), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, Release 11, in 3GPP TS 36.213, V11.4.0 (2013-September), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures, Release 11, and 3GPP TS 36.331, V11.5.0 (2013-September), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC), Release 11.
In existing LTE communication methods, a significant delay in accessing the medium may be incurred when networks are congested with multiple nodes contending for channel access.